When a vehicle is controlled to perform automatic travel control (auto cruise control), fuel economy is improved if a frequency of inertial travel is increased.
Therefore, as disclosed in JP-A-2012-219986 (Patent Document 1), a device for controlling the inertial travel of the vehicle so that a vehicle speed falls within a speed range, which is faster than a lower limit speed set to be slower than a preset target speed and is slower than an upper limit speed set to be faster than the target speed, has been suggested.
According to the device, when a slope changes from positive to negative, i.e., when the vehicle approaches a descending road, the inertial travel is enabled to start. Also, when the slope becomes positive and a slope change ratio becomes negative, it is determined that the vehicle is adjacent to a top. Also, a speed of the vehicle when controlled to perform the inertial travel in the vicinity of the top is estimated using a map in which a relationship between a shape of a travel road such as a slope, a slope change ratio and the like, a current vehicle speed, and a vehicle weight is preset. Also in a case where the estimated speed becomes equal to or greater than the lower limit speed, the inertial travel is enabled to start.
However, according to the device, even when the vehicle is controlled to perform the inertial travel, the speed of the vehicle may be decreased to become less than the set lower limit speed at an early stage due to influences of an air resistance, a rolling resistance and a slope resistance to be applied to the vehicle, depending on the slope of the descending road.
Also, according to the device, there is a case where the vehicle is not adjacent to the top even when the slope becomes positive and the slope change ratio becomes negative. In this case, even when the inertial travel is enabled to start, the speed of the vehicle decreases and becomes less than the set lower limit speed at an early stage.
Like this, if it is determined whether or not to start the inertial travel only based on the situations of the travel road such as the slope, the slope change ratio and the like of the travel road, a situation where the vehicle speed becomes less than the set lower limit speed at an early stage occurs in the above case. When this situation occurs, more fuels are consumed so as to release the inertial travel and to return to the target speed by consuming the fuel, so that the fuel economy is further deteriorated.